The cell is the unit of life. Probably, one of the fundamental characteristics of these is the capacity that these living beings have at the time of self-reproduction.
All cells reproduce by dividing into several daughter cells, which in turn can continue to proliferate. In the case that concerns us as humans, that is, in eukaryotic cells, there are two types of division: mitosis and meiosis. For this occasion, I will focus on the first of these and explain the phases of mitosis that it performs to carry out the formation of two daughter cells.
The common phase
The cells follow the pattern of a sequential process that concludes in cell division . This process is known as the cell cycle. In short, the cycle consists of preparing the cell for its imminent splitting into two. This process has traditionally been divided into two major phases: the interface and the M phase. The interface is shared in both mitosis and meiosis.
If the eukaryotic cell cycle took 24 hours, the interface would occupy 23 of these, leaving only one hour for division. It is normal that it takes so long, since during this stage the cell doubles its size, doubles its genetic content and prepares the necessary tools for everything to go well in the formation of new cells.
The interface is generally divided into three stages:
- G1 phase (Gap1): the cell grows in size and is metabolically active .
- S Phase (Synthesis): the cell replicates its DNA.
- G2 phase: the cell continues to grow and synthesizes proteins that will be used for mitosis .
Once the cell enters the S-phase, there is no return to the process of division unless its DNA is detected as damaged. Cells have signaling systems that allow the recognition of their DNA and if something goes wrong, they can stop the process so as not to cause further problems. If all is well, the cell is already prepared for its imminent proliferation.
Phases of mitosis
After completing the interface, the cell enters the M phase with the aim of forming new cells . Mitosis results in two sister cells, of equal genetic content. Mitosis has differences according to the eukaryotic cell that performs it, but they all have in common the condensation of the chromosomes, the formation of the mitotic spindle and the union of the chromosomes to the latter… many new concepts that I will clarify.
Traditionally, mitosis has been divided into four distinct stages: prophase, metaphase, anaphase and telophase. To explain this process I will focus on the case of human cells.
At the start of M Phase, the replicated DNA that is entangled condenses into a more compact form known as a chromosome . In the case of humans we have 23 chromosomes. As they are still preparing to divide, the chromosomes are still formed by the two chromatids (the original and the copy), joined by a midpoint known as the centromere, giving the typical image of an X.
Not only this happens; it is worth remembering that the genetic material is found inside a nucleus , and in order to be able to access it, the membrane that surrounds them must be degraded. In addition, the mitotic spindle is generated, a set of filamentous protein structures (microtubules), which will later act as transport pathways for the chromosomes.
When these microtubules mentioned above attach to the centromere of the chromosomes and line up right in the center of the cell is when the metaphase occurs. You are now at the point where the genetic content is separated. This is a phase of mitosis that is rapid.
In this phase of mitosis you will understand how the mitotic spindle acts. What it does is separate the sister chromatids and drag them to opposite poles, as if they were a fishing rod that is picking up the line. In this way, the same genetic content is achieved in the two new cells.
Once on opposite sides, the chromosomes are decondensed in their usual form and the nucleus that contains them is regenerated. This is accompanied by cytokinesis, i.e. the splitting into two cells . This process begins at the end of the anaphase, and consists in the case of animal cells in a contractile ring that strangles the cell membrane more or less by the center, as if it were a balloon, until two independent cells are generated.
The end result of mitosis is the formation of two interphase sister cells, since they contain the same genetic content and there has been no modification of this, has simply been replicated . It should be noted that any abnormality in this process stops it immediately.